Why High Pressure Washing Damages Your Roof

Every week someone asks me about high pressure roof cleaning. Usually they've seen a neighbour get it done or found a cheap quote online. My answer is always the same: don't do it. Here's why.

What high pressure actually does to roof tiles

Standard roof tiles — whether clay, concrete, or slate — are not smooth. They have a textured surface and, in the case of concrete tiles, a protective granule coating that keeps the tile waterproof and UV-resistant. High pressure water (anything above 1,000 PSI aimed at a roof) strips those granules clean off.

Once the granule coating is gone, the tile becomes porous. Water soaks in, freezes in winter, expands, and the tile cracks from the inside out. You won't see the damage immediately — it typically shows up 12–24 months later as a slow leak you can't trace.

High pressure also dislodges ridge tiles, blows out the mortar bedding underneath them, lifts flashing, and forces water directly under the tile line. On an older roof, you might lose 20–30 tiles just from the force of the water.

It will void your home insurance

This is the part most people don't find out until it's too late. Most UK home insurance policies contain a clause excluding damage caused by "improper maintenance" or "non-approved cleaning methods." High pressure roof cleaning — especially if performed by an uninsured operator — almost always falls under that exclusion.

If your roof leaks six months after a high pressure clean, your insurer will ask how it was maintained. If you can't show that approved methods were used by an insured contractor, your claim will be rejected.

How low-pressure cleaning actually works

The correct process uses a pressure washer running at low pressure — well under 300 PSI. That's enough to clean the roof properly without cracking tiles or stripping their protective surface. It's still a pressure washer, just one running at a controlled, safe pressure rather than the full 2,000+ PSI of a standard jet wash.

After the roof is cleaned, we flush out the gutters — removing debris, standing water, and anything the roof wash has knocked loose. Then we apply a specialist biocide chemical treatment to the clean roof surface. The biocide penetrates tile pores and kills remaining algae and moss spores at the root, leaving a residual treatment that prevents regrowth for 3–5 years.

Done in the right order — wash, gutters, biocide — the result is a clean roof that stays clean. High pressure skips the biocide step entirely, which is why the moss is back within 12 months.

Our 3-year guarantee

We back every roof clean with a 3-year guarantee. If any moss returns within that period, we retreat it at no charge. We also include a tile replacement guarantee — if any tile breaks during our work (it won't, with low-pressure biocide treatment), we replace it.

No high pressure operator can make that offer. Because they know what their method does.